135
Upper Limit Temperature Protection
MICROTEMP® thermal cutoffs from Therm-O-Disc offer an accurate, reliable solution to the need
for upper limit temperature protection. Known as a thermal fuse, thermal link, or TCO, the
MICROTEMP®thermal cutoff provides protection against overheating by interrupting an electrical
circuit when operating temperatures exceed the rated temperature of the cutoff.
MICROTEMP®Features:
•One-shot operation cuts off electrical power
•Current interrupt capacity up to 25 amps @ 250VAC
•Low resistance
•Compact size
Operating Principle of the MICROTEMP®TCO
The active trigger mechanism of the thermal cutoff is an exclusively formulated, electrically
nonconductive pellet. Under normal operating temperatures, the solid pellet holds spring-loaded
contacts closed.
When a predetermined temperature is reached, the pellet melts, allowing the compression spring
to relax. The trip spring then slides the contact away from the lead and the circuit is opened (see
figures 1 and 2).
After a MICROTEMP®thermal cutoff opens a circuit, the TCO needs to be replaced. This
replacement procedure must include correction of the fault condition before the product is
operated again.
MICROTEMP®Thermal Cutoffs:
INTRODUCTION
136
THERMAL
PELLET
COMPRESSION
SPRING
TRIP SPRING
CERAMIC
BUSHING
CASE AND LEAD
ASSEMBLY
DISCS
STAR CONTACT
ISOLATED
LEAD
Before Operation
Grey area shows
current path
EPOXY
SEAL
COMPRESSION
SPRING
TRIP SPRING
CERAMIC
BUSHING
CASE AND LEAD
ASSEMBLY
DISCS
STAR CONTACT
ISOLATED
LEAD
After Operation
Grey area shows opened or
broken current path
EPOXY
SEAL
MICROTEMP®G4, G6 & G7 Series TCO
THERMAL
PELLET
COMPRESSION
SPRING
TRIP SPRING
CERAMIC
BUSHING
CASE AND LEAD
ASSEMBLY
DISCS
STAR CONTACT
FLOATING
CONTACT
Before Operation
Grey area shows
current path
ISOLATED LEAD
EPOXY
SEAL
COMPRESSION
SPRING
TRIP SPRING
CERAMIC
BUSHING
CASE AND LEAD
ASSEMBLY
DISCS
STAR CONTACT
ISOLATED LEAD
After Operation
Grey area shows opened or
broken current path
FLOATING
CONTACT
EPOXY
SEAL
MICROTEMP®G5 & G8 Series TCO
Figure 1
Figure 2
137
MICROTEMP®thermal cutoffs are available in a range of temperatures and electrical ratings to
meet application requirements (see figure 3). There are five primary types of thermal cutoffs
available. Standard dimensions of each TCO series are shown in figure 4.
G4 Series
Rated for continuous operating currents up to 10 amps @ 250VAC (15 amps @ 120VAC), the G4
series MICROTEMP®TCO is the industry standard for over-temperature protection. The G4 series is
applied to millions of appliances and personal care products each year, providing reliable back-up
protection for temperature controlling thermostats and other over-temperature conditions. The
G4 series is also widely applied in office machines, portable heaters and industrial equipment as a
thermal safeguard.
G5 Series
Designed for higher current applications, the G5 series MICROTEMP®TCO is rated for operating
currents up to 16 amps @ 250VAC (20 amps @ 250VAC and 25 amps @ 120VAC at UL/CSA).
Similar in appearance to the G4 series, the G5 series has a different internal construction designed
for interrupting higher currents.
G6 Series
The G6 series MICROTEMP®TCO can be utilized in applications where a higher maximum-overshoot
temperature rating is not required, yet it is rated for operating currents up to 16 amps @ 250VAC.
It is the same physical size as the G4, G5 and G8 series TCOs.
G7 Series
The G7 series MICROTEMP®TCO is designed to satisfy applications requiring miniaturized components
that do not need maximum current interrupt capability. The G7 is just 2/3 the size of the G4 and
G5, and with a current interrupting capability of 5 amps @ 250VAC, it is capable of meeting the
requirements of transformers, motors, battery packs and electronic circuit applications.
G8 Series
Designed for very high-current applications such as major appliances and high-wattage electric
heat packages, the G8 series MICROTEMP®TCO is rated for operating currents up to 25 amps @
250VAC. More economical than electromechanical bimetal-type one shot devices, it can be
utilized in applications where its small size is an advantage in terms of mounting (it’s the same
physical size as the G4, G5 and G6 series TCOs) and thermal response.
MICROTEMP®Thermal Cutoffs:
TYPES & SPECIFICATIONS
138
MICROTEMP®TCO Operating Temperature Summary
Maximum Overshoot Temperature
T
m°C T
m°C T
m°C T
m°C T
m°C T
m°C T
m°C
G4 Series G5 Series G6 Series G7 Series G8 Series R9 Series R7 Series
072 47 100 175 100 — 175 100 —
077 52 125 200 125 125 200 125 125
084 59 125 200 125 125 200 125 125
093 68 140 215 — 140 215 140 140
098 73 140 215 140 140 215 140 140
104 79 150 225 150 — 225 150 —
110 85 150 225 — 140 225 150 140
117 92 160 235 160 140 235 160 140
121 96 160 235 160 150 235 160 150
128 103 160 235 160 150 235 160 150
144 119 175 250 175 175 250 175 175
152 127 175 250 175 175 — 175 175
167 142 210 285 — 200 285 210 200
184 159 210 350 210 200 350 210 200
192 167 210 350 210 — 350 210 200
216 191 375 375 — — — 375 —
229 200 375 375 375 — 375 375 —
240 200 375 375 375 — 375 375 —
Max. Holding
Open Temp
Temp Th
T
f°C °C
TM–Maximum overshoot
temperature: temperature
up to which TCO will not
change status
TF–Functioning open
temperature
tolerance: +0, -5°C
TH–Holding temperature:
Maximum continuous
exposure temperature
C.T.I. – Comparative tracking
index (all primary thermal
cutoffs): 250VAC
NOTE: G4, G5, G6,G7 and G8
series TCOs with TF ≥184°C
comply with UL conductive
heat aging (CHAT)
requirements.
`Dimensions – Inches (millimeters) G4, G5, G6 & G8 Series G7 Series
Standard AOverall Length ± .12 (±3.0) 2.51 (63.8) N/A
Leads BEpoxy Lead Length (Reference) 0.55 (14.0) N/A
CCase Lead Length ± .06 (± 1.5) 1.38 (34.9) N/A
AOverall Length ± .12 (±3.0) 3.26 (82.8) 3.26 (82.8)
Long Leads B Epoxy Lead Length (Reference) 1.30 (33.0) 1.50 (38.1)
CCase Lead Length ± .06 (± 1.5) 1.38 (34.9) 1.38 (34.9)
DCase Lead Diameter .040 (1.0) .023 (.57)
Lead Material D Case Lead Material Tin-Plated Copper Tin-Plated Copper
and Diameter E Epoxy Lead Diameter .040 (1.0) .023 (.57)
EEpoxy Lead Material Silver-Plated Copper Silver-Plated Copper
Case F Case Length (Reference) .58 (14.7) .38 (9.6)
Dimensions G Case Diameter (Reference) .158 (4.0) .118 (3.0)
MICROTEMP®TCO
Standard Dimensions
Figure 3
Electrical Rating Summary
Maximum Overshoot Temperature
G4 Series G5 Series G6 Series G7 Series G8 Series R9 Series R7 Series
Resistive Inductive Resistive Resistive Resistive Inductive Resistive Resistive Resistive
UL/CSA 10A/250VAC 8A/250VAC 16A/250VAC 16A/250VAC 5A/250VAC 4.5A/250VAC 25A/250VAC — —
15A/120VAC 14A/120VAC 25A/120VAC 5A/24VDC 4.5A/120VAC
21A/240VAC
IEC 10A/250VAC 8A/250VAC 16A/250VAC 16A/250VAC 5A/250VAC 4.5A/250VAC 25A/250VAC — —
15A/120VAC 14A/120VAC 4.5A/120VAC
METI 10A/250VAC — 15A/250VAC 15A/250VAC 5A/250VAC ——15A/250VAC 7A/250VAC
5A/24VDC 7A/24VDC
Agency
Figure 4
139
Packaged Thermal Cutoffs
Therm-O-Disc also offers a variety of packaging options for MICROTEMP®thermal cutoffs.
Therm-O-Disc offers two standard packages with wide application in the HVAC industry. Primarily
designed for heating applications, the GXAM04 and GXAM06 packages mount a standard G4 or
G5 TCO in a high temperature ceramic base (see figure 5).
The popular GXAP Potted TCO packages consist of a TCO epoxy-potted into a plastic insulating
mounting case. The assembly can be supplied with various case materials, shapes and terminations.
They can be used to accurately sense temperature as probes and as surface, airstream and ambient
sensors (see figure 6). They can be easily replaced in the field without disturbing the rest of the circuit.
Custom packages, insulations or special assemblies may be specified to meet specific unique
application needs. By taking advantage of our technical expertise and high volume production
methods and equipment, we can provide significant savings on custom packages.
Packaged TCO Material Specifications
Figure 5
Figure 6
GXAM04
GXAM06
GXAP04
GXAP02
GXAP12
GXAP05
GXAP08
Type Base Material Material Rating Maximum TCO
Temperature °C Temperature °C
GXAP01 Valox DR48 120 128
GXAP02 Ryton R-4 220 152
GXAP04 Valox DR48 120 128
GXAP05 Valox DR48 120 128
GXAP08 Ryton R-7 220 184
GXAP10 Ryton R-7 220 184
GXAP12 Valox DR48 120 128
GXAM04 Ceramic DIN VDE 0335, C 221 >250 240
GXAM06 Ceramic DIN VDE 0335, C 221 >250 240
140
Lead Configurations
The MICROTEMP®TCO can be furnished with virtually any lead configuration specified for an
application. Lead curls are available to match most sizes of screws along with varying lead lengths
and lead forms.
All types of terminations, such as quick connects, ring terminals and blade terminals, are available
at additional cost. In addition, tape and reel packaging can be specified to meet high volume
requirements.
Temperature Ratings
MICROTEMP®thermal cutoffs are available in a wide range of opening temperatures, providing
designers a high degree of flexibility (see figure 3). Determining the correct TCO temperature
calibration requires significant application testing.
The proper calibration will be affected by application variables such as I2R self heating of the TCO,
heat transfer through insulation and heat dissipation due to heat sinking and air flow.
Thermocoupled “dummy” TCOs, that match the physical and electrical characteristics of a
functional TCO, are available to help evaluate application specific variables.
For more information on testing and installing MICROTEMP®TCOs, please review the
MICROTEMP®thermal cutoff technical information section beginning on page 143.
Direct Current (DC) Applications
MICROTEMP®thermal cutoffs do not have published electrical ratings for direct current (DC)
applications. Current interruption capacity in DC circuits is highly application sensitive.
Therm-O-Disc recommends thorough testing of DC electrical applications using the testing
guidelines in Therm-O-Disc’s MICROTEMP®thermal cutoff technical information section.
Samples and Quotations
MICROTEMP®TCO samples and thermocoupled “dummies” are readily available for determining
the correct response and desired performance in an application. For more information on
MICROTEMP®TCOs, call a Therm-O-Disc sales engineer at 419-525-8300.
141
Lead Cutting
`Dimension A Dimension B Dimension C
0.95 (24.2) 0.22 (5.6) 0.73 (18.6)
DIM. A
DIM. C DIM. B
Minimum Dimensions
Inches (millimeters)
Tape and Reel Packaging
`Item Dim. A Dim. B Dim. C Dim. D Dim. E Dim. F
GXAA0900TTTC 1.66 (42.1) 2.80 (71.1) 1.38 (35.1) 3.26 (82.8) 3.60 (91.4) 0.200 (5.1)
G7FA0900TTTC 1.66 (42.1) 2.80 (71.1) 1.38 (35.1) 3.26 (82.8) 3.60 (91.4) 0.197 (5.0)
F0.060(1.5)
DIM. A
DIM. B
DIM. F
F
0.031(0.8)
(PITCH MEASURED AT TAPE ON MICROTEMP SIDE)
DIM. D
DIM. C
Dimensions – Inches (millimeters)
NOT TO SCALE
DIM. E
TCO ORIENTATION
SPOOL ROTATION
REF.
Ø 0.026(0.7)
REF.
Ø 0.551(14.0)
REF.
Ø 1.25(3.2)
TAPING CORRUGATION
THIS SIDE
142
Product Nomenclature
MICROTEMP®TCO
MICROTEMP®TCO Packages
Figure 7
As shown in figure 7, Therm-O-Disc MICROTEMP®TCOs follow a consistent product nomenclature that identifies the
basic product type, lead wire size, special features and packaging options. For example, a standard G4 series TCO
calibrated to open at 192°C would have a part number G4A00192C.
MICROTEMP®TCO Product Markings
Primary TCOs Secondary Packages
XXXXXXXX Special customer identification XXXXXXXXX Special customer identification
(when required, up to 9 characters) (when required, up to 9 characters)
MICROTEMP®Registered trademark MICROTEMP®Registered trademark
P ZZZZZ (P) Manufacturing plant; (may be T-O-D)
(ZZZZZ) Date code G Z X X XX RR Part number (see figure 7)
G Z X XX Part number (see figure 7) TFTTTC P ZZ (TFTTTC) Maximum open temperature °C;
TFTTTC () Underwriters Labs logo; (P) Manufacturing plant location;
(TFTTTC) Maximum open (ZZ) Manufacture date code;
temperature °C ( ) Underwriters Labs logo
G Z X X XX RR TTTC
G – Rating type
(G – Global, Y-Non Agency)
Z – Internal construction
(4, 5, 6, 7, 8, 9)
X
– Case material and lead wire
(A, C, D, F)
TTTC – Maximum
open temperature in °C
RR – Assembly modifications
(Plating, terminal bend, stenciling)
XX – Specific package construction
(00-99)
X – General packages TCO type
(Configuration, potted, mounting base, etc.)
G Z X XX TTTC
G – Rating type
(G – Global, Y – Non Agency, R – Regional)
Z – Internal construction
(4, 5, 6, 7, 8, 9)
TTTC – Maximum
open temperature in °C
XX – Modification of basic TCO
(Plating, lead material,
lead length, stenciling)
X – Case material and lead wire
(
A, C, D, E, F
)
14/09/2010
Farnell,
I have the information you asked for.
G4A00 has one short lead and one long lead.
G4A01 has 2 long leads.
We supply the G4A01 to Farnell so that the customer has the choice and
can trim the lead if required.
Let me know if I can help further
Best regards,
Foremost Electronics Ltd
14 Bluegate Business Park
Great Bardfield
Essex
CM7 4PZ
TEL: +44 (0) 1371 811171
FAX: +44 (0) 1371 810933
143
MICROTEMP®thermal cutoffs, available in a variety of standard and custom configurations, pro-
vide reliable one-shot, over-temperature protection in a wide range of applications. Performance
can be affected by installation method and proper location of the thermal cutoff. Both application
and installation are important in the overall performance of the product, and thorough testing is
necessary for both AC and DC applications. The following guidelines will answer most questions
concerning these two subjects.
Application of Thermal Cutoffs
A thermal measurement procedure that utilizes a “dummy” thermal cutoff can assist in determining
the appropriate calibration temperature and design location of MICROTEMP®thermal cutoffs. The
dummy matches the electrical characteristics of the thermal cutoff but does not have thermally
responsive parts. The dummy is supplied with a thermocouple attached to the case of the thermal
cutoff (see figure 8).
Figure 8
View of required thermocouple attachment before soldering
Dummy cutoffs can be supplied with Type J, Type T or Type K thermocouples. Other thermocouple
types can usually be supplied upon request at a nominal charge.
Location
Sufficient time and effort must be used to determine the proper and most desirable location for a
thermal cutoff. The employment of infrared thermography, or a sufficient number of thermocouples
to identify the highest temperature areas in the product requiring protection during fault conditions,
should be considered.
MICROTEMP®Thermal Cutoffs:
TECHNICAL DATA
144
Calibration Temperature
It is necessary to select a thermal cutoff rating above the maximum temperature experienced during
normal operation, including expected short-term temperature overshoots. The temperatures
experienced by the thermal cutoff during normal operation will determine the life expectancy for
the thermal cutoff. If the thermal cutoff rating is too close to the temperature experienced during
normal operation (including overshoot temperatures after opening of a thermostat, etc.), the
probability of a nuisance trip increases.
Nuisance trips are caused by pellet shrinkage due to repeated operation at temperatures near but
below calibration temperature, or excessive thermal gradients across the case of the TCO and its
leads (see “Thermal Gradients”). More information on nuisance tripping due to pellet aging is
available in U.L. Standard 1020, under the section on Thermal Element Stability Test. Therm-O-Disc
has compiled standard life curves by subjecting MICROTEMP®thermal cutoffs to very controlled
temperatures for extended time periods under ideal laboratory conditions. Therefore, these
standard life curves should be used only as a guideline.
Comparison of measured temperatures to MICROTEMP®thermal cutoff standard life curves should
not replace customer life testing using functional thermal cutoffs for the particular application.
The design engineer must make the trade-off between response and life of the TCO based on
product requirements. It is important to remember that temperatures experienced in actual
application will vary from unit to unit.
Test Procedure
Install the dummy cutoff in the electrical circuit to be opened in the event of a fault condition.
Position it in the area that has been selected to be protected within the product based on prior
determinations of the maximum permissible temperatures to be allowed. The dummy cutoff
should be installed using the same mounting and electrical connection that will be used for
functional TCOs in production. Connect the thermocouple leads to a digital temperature
measuring device to record temperatures. The product to be protected can now be operated,
and the normal operating temperature monitored. Note that the thermal cutoff dummy is not a
functional TCO and therefore will not open the circuit in the test setup.
145
Figure 9
Figure 9 illustrates a typical installation of a thermocoupled cutoff. Note that the body of the thermal
cutoff is at the same potential as the connecting circuit; therefore, it must be electrically isolated
from the surface against which the cutoff is mounted. Also note that the thermocouple wire is at
the same potential as the connecting circuit.
CAUTION . . . To avoid a false reading of the unit under test, thermocouple wires must not make
contact with each other except at the temperature sensing junction.
CAUTION . . . Ensure that the thermocouple wire insulation will provide isolation against short-
circuiting and shock hazards.
CAUTION . . . The terminal of the temperature measuring instrument, to which the thermocouple
is attached, will be at the same potential as the connecting circuit wire. This instrument must be
electrically isolated and considerable caution must be exercised in its use, since one of the
thermocouple terminals is frequently grounded to the instrument chassis.
Before using measuring equipment powered directly from standard line voltages, check operation
manuals. Be sure line voltages impressed on the thermocouple wires by the dummy cutoffs will
not cause damage to the instrument.
The more closely the actual operating and ambient conditions can be simulated during test, the
more valid the test results will be. These tests are necessary to empirically include the variable
factors that need to be considered to select the properly rated thermal cutoffs. These factors
include, but are not limited to, the heating effect of the current through the cutoff, adjoining
THERMAL CUTOFF "DUMMY"
TERMINAL OR SPLICE
ELECTRICAL INSULATOR
CONNECTNG CONDUIT WIRE
THERMOCOUPLE
TEMPERATURE OR VOLTAGE MEASURING DEVICE
146
terminals and leads, heating or cooling effect of the terminals and external leads, rate of
temperature rise, air flow, shock, vibration and other environmental and operating conditions
unique to the application.
The product and application being tested will determine the number of cycles that must be run to
determine the maximum “normal” operating temperature. “Overshoot” temperatures should be
included in the determination of the maximum “normal” operating temperature. The overshoot
temperature is often considerably higher than the temperature reached at the moment the
thermostat opens. The conclusion of these tests will provide the maximum “normal” operating
temperature at the thermal cutoff (at maximum anticipated voltage, ambient temperature, etc).
The overshoot temperature seen by the thermal cutoff after the thermal cutoff opens in the
application must also be carefully examined.
Manufacturing tolerances and variations should be carefully considered, and a sufficient number
of units evaluated, to provide a statistical basis on which to determine the operating overshoot
temperatures.
After obtaining the above information, test the product under fault conditions and monitor to
determine that desired fault condition temperatures are not exceeded.
Where there are a variety of fault conditions, (e.g., short-circuited thermostats and transformer
secondaries, locked motor rotors and solenoids, high ambient temperatures, restricted or
blocked airflow, etc.), consideration should be given to multiple fault conditions which could
occur simultaneously during the lifetime of the product, and to faults which may cause localized
overheating in areas away from the TCO.
When the fault conditions have been set up, note the temperature of the dummy cutoff when the
maximum desired temperature limit is reached. At this point the circuit is manually interrupted.
This test should be run several times, in several different units. In some applications, it will not be
possible to “save” the tested item from damage, but only prevent the product from creating an
external fire or electrical hazard. Damaged products should not be retested, since the results may
not be the same as with undamaged units. The MICROTEMP®thermal cutoff ratings selected
should be equal to or less than the temperature recorded at the dummy thermal cutoffs at the
time the maximum desired temperature is reached.
CAUTION . . . Excessive overshoot temperatures after the opening of the thermal cutoff may cause
dielectric breakdown of the thermal cutoff and allow reconduction to occur. Functional thermal
cutoffs should be tested to verify proper operation of the thermal cutoffs in the application (see
figure 3).
147
Substitute actual thermal cutoffs in a sufficient number of finished products and re-run the
tests to obtain statistical verification of the results. For multiple TCO applications, test functional
thermal cutoffs under fault conditions so that the product overheats and each thermal cutoff is
independently called upon to interrupt the flow of current. Each thermal cutoff should open the
circuit independently of any other over-temperature limit controls, with product damage not
exceeding an acceptable level. This test should be run using the maximum voltage and current;
the thermal cutoff will be expected to interrupt and hold open.
Installation of Thermal Cutoffs
The performance of a MICROTEMP®thermal cutoff can be affected by installation methods such
as soldering, welding, splicing, lead bending, insulation, clamping and mounting. Certain precautions
should be taken during installation to ensure that the MICROTEMP®thermal cutoff is not damaged,
which may cause it to not operate in its intended manner. Likewise, care should be taken during
installation to ensure that the TCO in every unit experiences the expected temperature range
environment previously determined during the calibration temperature selection. The following
guidelines should be used to minimize undesirable conditions that can result from improper
installation practices.
Soldering Leads
Thermal cutoff leads should be heat sinked during the soldering operation (see figure 10). If
excessive heat is conducted by the leads into the thermal cutoff, it can shorten the life of the TCO.
In addition, excessive lead temperatures can damage the epoxy and possibly result in the TCO
failing to open. More heat sinking is necessary for thermal cutoffs with low temperature ratings.
Figure 10
HEAT SINK HERE
SOLDER SOLDER
148
Test samples should be x-rayed before and after the soldering operation. The size of the chemical
pellet should be measured with an optical comparator or a toolmaker’s microscope to verify that
no shrinkage has occurred during the soldering operation (see figure 11). The epoxy seal should
retain its size and shape and not discolor. If the chemical pellet or the epoxy have changed size as
a result of the soldering operation more heat sinking is required.
Figure 11
Welding Leads
The thermal cutoff leads may also need to be heat sinked during a welding operation (see figure
12). The same precautions and tests described in the soldering section should also be followed for
welded leads.
Figure 12
To avoid damaging or welding internal parts, care should be taken that none of the welding current
is conducted through the TCO. A welding current of hundreds of amperes can weld the internal
parts together, resulting in the TCO failing to open.
TCO leads must be supported during the weld operation to prevent breaking the thermal cutoff
epoxy seal.
WELD POINTS
HEAT SINK HERE
EPOXY SEAL BARREL
SPRING
THERMAL PELLET
ISOLATED
LEAD
TRIP
SPRING
CASE &
LEAD ASSEMBLY
149
Splices & Terminals
Insecure splices and terminations may produce high resistance junctions which can cause self
heating (I2R) as power is dissipated across these junctions during product operation.
Heat from these hot spots can flow down the thermal cutoff leads and increase the temperature
of the thermal cutoff (see figure 13). Nuisance openings of the thermal cutoffs or degradation of
the epoxy seal can occur as a result of the heat generated by high resistance junctions. The splice
or termination junction may initially measure low resistance, but can change to a much higher
resistance after several temperature cycles. It is generally better to splice MICROTEMP®thermal
cutoff leads to stranded lead wires rather than solid wires as the stranded wire may be crimped
tighter and maintain better electrical contact during temperature cycling.
Figure 13
The temperature capabilities of the splice and/or termination should be considered. For example,
solder back-up should be considered for splices of terminations in applications cycled at temperatures
exceeding 150°C.
Bending Leads
When configuring leads, special care must be exercised in supporting the leads at each end near
the body of the thermal cutoff so that the case will not be distorted or the epoxy will not be
cracked or broken. At least 0.125” (3mm) should be maintained between the epoxy seal and any
lead bends (see figure 14).
SPLICE HEAT FLOW HEAT FLOW
NUT
LOCK
WASHER
BOLT
SPLICE
TERMINATION
SPLICE
x-ref colour against temperature
150
Figure 14
Dimensions are shown in inches (millimeters).
Thermal Gradients
Ideal TCO placement subjects the entire TCO case, leads, epoxy seal and internal components to a
uniform temperature environment.
Care should be exercised in the placement of the TCO to minimize thermal gradients across the
TCO body. In certain applications, the TCO can be mounted in a position where heat is conducted
to the body of the TCO through one of the leads, resulting in thermal gradients across the TCO.
Over time, the TCO life can be reduced by thermal gradients if the isolated (epoxy) lead is at a
consistently lower temperature than the case lead. Long term testing is recommended in
determining whether these conditions exist in the application.
To minimize the effects of thermal gradients and the temperature increase of the TCO body from
this heat flow, attach the isolated (epoxy) lead, rather than the case lead, to the heat source.
TCO dummies can be supplied with thermocouples on both ends to facilitate gradient evaluations.
Temperature Limits
The temperatures experienced during normal operation, including expected temperature overshoots,
will determine the life expectancy of the TCO. Nuisance trips can result if the thermal cutoff rating
is too close to the temperatures experienced during normal operation. Thermal cutoffs of any
temperature rating should not be subjected to continuous normal temperatures in excess of
200°C. Additionally, overshoot temperatures after the opening of the thermal cutoff should be
minimized to avoid dielectric breakdown and reconduction of the thermal cutoff.
.09 MIN.
(2.3 mm)
.04 MIN. RAD.
(1.0 mm)
.70 MIN. (17.8 mm)
151
CAUTION . . . The thermal cutoff may fail to open the electrical circuit under certain conditions.
Distortion of the case, breaking or cracking the seal, exposing the epoxy seal to cleaning solvents,
compression of the leads and current surges that exceed the operating specifications of the thermal
cutoff may cause the thermal cutoff not to open. In addition, pellet shrinkage due to thermal
aging under some circumstances may also result in failure to open. Finally, a very low rate of
temperature rise may produce conditions that may also result in failure to open. Care must be
taken to avoid any mishandling or misapplication of the thermal cutoff.
CAUTION . . . Although TCOs are highly reliable devices, a TCO may fail to open in operation for
one or more of the reasons set forth above. These conditions must be taken into account by the
product design engineer in determining the level of reliability needed for the application. If failure
of the TCO to open could result in personal injury or property damage, the product design engineer
may want to consider using one or more redundant TCOs of different ratings to achieve the
desired level of reliability. A number of consumer product design engineers have incorporated
redundant TCOs of different ratings in their designs for this reason.
Definition of Terms
Maximum Open Temperature or Rated Functioning Temperature (Tf, TF):
The maximum temperature at which the thermal cutoff changes its state of conductivity to open
circuit with detection current as the only load. The rated functioning temperature is measured
during a temperature rise of approximately 0.5°C per minute.
Holding Temperature (Th, TH):
The maximum temperature at which, when applying the rated current to the thermal cutoff, the
state of conductivity will not change during a period of one week.
Maximum Overshoot Temperature or Maximum Temperature Limit (Tm, TM):
The maximum temperature at which the thermal cutoff, having changed its state of conductivity,
can be maintained for a specified period of time, during which its mechanical and electrical
properties will not be impaired.
Rated Voltage:
The maximum voltage that can be applied to the circuit in which the thermal cutoff is used.
Rated Current:
The maximum current that the thermal cutoff is rated to interrupt at the rated voltage.
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Agency Recognition
MICROTEMP®thermal cutoffs are recognized by the following major agencies:
UL BEAB METI CSA VDE
Underwriters British Ministry of Canadian Varband
Laboratories Inc. Electrotechnical Economy, Trade Standards Association Deutscher
(USA) Approvals Board and Industry of Electrotechniker e.V.
Japan (F. R. G.)
MICROTEMP®thermal cutoffs are recognized by the major approval agencies throughout the world
for AC circuit applications (they do not have recognition for DC circuit applications). These agency
electrical ratings can be used as a guideline when evaluating specific thermal cutoff applications.
However, the electrical and thermal conditions to which the thermal cutoff may be exposed in an
application may differ significantly from agency test conditions. Accordingly, customers should
not rely solely on agency ratings but rather must perform adequate testing on the particular
application to confirm that the TCO selected is appropriate for that application and will operate as
intended.
Important Notice
Users must determine the suitability of the control for their application, including the level of
reliability required, and are solely responsible for the function of the end-use product.
These controls contain exposed electrical components and are not intended to withstand
exposure to water or other environmental contaminants which can compromise insulating
components. Such exposure may result in insulation breakdown and accompanying localized
electrical heating.
A control may remain permanently closed or open as a result of exposure to excessive mechanical,
electrical, thermal or environmental conditions or at normal end-of-life. If failure of the control to
operate could result in personal injury or property damage, the user should incorporate supplemental
system control features to achieve the desired level of reliability and safety. For example, backup
controls have been incorporated in a number of applications for this reason.
PS
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